A fully functioning forest has a great capacity to regenerate. But exhaustive hunting of tropical rainforest wildlife
can reduce those species necessary to forest continuance and regeneration. For example, in Central Africa, the
loss of species like gorillas, chimps, and elephants undercuts the seed dispersal and slows the recovery
of damaged forest.
Loss of habitat in the tropics also affects the regeneration of temperate species. North American
migratory birds, important seed dispersers of temperate species, declined 1-3 percent annually from 1978-1988.
INCREASE OF TROPICAL DISEASES
The emergence of tropical diseases and outbreaks of new diseases, including nasty hemorrhagic fevers like ebola
and lassa fever, are a subtle but serious impact of deforestation. With increased human presence in the rainforest,
and exploiters pushing into deeper areas, humans are encountering microorganisms with behaviors unlike
those previously known. As the primary hosts of these pathogens are eliminated or reduced through forest disturbance
and degradation, disease can break out among humans. Although not unleashed yet, someday one of these microscopic
killers could lead to a massive epidemic as deadly for our species as we have been for the species of the
rainforest. Until then, local populations will continue to be menaced by mosquito-borne diseases like dengue fever,
Rift Valley fever, and malaria, and water-borne diseases like cholera.
Many emergent and resurgent diseases are directly linked to land alterations which bring humans in closer contact
with such pathogens. For example, malaria and snailborne schistosomiasis have escalated because of the proliferation
of artificial pools of water like dams, rice paddies, drainage ditches, irrigation canals, and puddles created
by tractor treads. Malaria is a particular problem in deforested and degraded areas, though less so in forested zones where there
are few stagnant ground pools for mosquito breeding. These pools are most abundant in cleared regions and areas
where tractors tear gashes in the earth.
Malaria — which is estimated to infect 300 million people a year worldwide, killing 1-2 million — is a major threat to forest-dwelling indigenous peoples who have developed little or no (in the case of uncontacted tribes) resistance to the disease and lack access to antimalarial drugs. Malaria in the 1990s was cited for killing an estimated 20 percent of the Yanomani in Brazil and Venezuela. Drug-resistant forms of malaria means the disease is again becoming a threat in places where it was thought to be under control. Models suggest that climate change could expand the distribution of malaria-carrying mosquitos.
The outbreak of disease in the tropics does not affect only the people of those countries, since virtually any
disease can be incubated for enough time to allow penetration into the temperate developed countries. For example,
a Central African doctor infected with the ebola virus from a patient can board a plane and land in London within 10
hours. The virus could quickly spread among the city's large population Additionally,
every person at the airport who is exposed can unknowingly carry the pathogen home to their native countries around
the world.
According to the Centers for Disease Control (CDC) in Atlanta, deaths from infectious disease are on the rise.
Infectious disease is the leading cause of death worldwide and the third leading cause of death in the United States. Infectious disease have had a major
role in human mortality throughout history. At least one-third of human deaths during World War I came from an
infectious disease: influenza. In 1919, between 20 million and 100 million died from the flu—more than the number of total casualties
from the war.
Slash-and-burn agriculture in the rainforest of Borneo. Click image for more pictures. (Photo by R. Butler)
Review questions:
How is deforestation linked to the emergence of disease?
The decline in North American migratory birds over the 1978-1988 period is reported by the U.S. Fish and Wildlife Service in its Breeding Birds Survey 1990 and further detailed in Terborgh, J.W., Where Have All the Birds Gone? Essays on the Biology and Conservation of Birds that Migrate to the American Tropics, Princeton: Princeton University Press 1989.
H.J. Van der Kaay discusses the threat of emerging pathogens resulting from increased forest loss and contact with primary disease hosts in "Human diseases in relation to the degradation of tropical rainforests," Rainforest Medical Bulletin, Vol. 5, no. 3, Dec. 1998.
In her work, The Coming Plague (New York: Farrar, Straus, and Giroux, 1994), L. Garrett reviews the gamut newly emerging diseases and suggests the importance of deforestation in bringing some pathogens in closer contact with human populations. For a popular and thrilling account of one such virus, the hemorrhagic Ebola virus, read R. Preston's The Hot Zone (New York: Random House, 1994). S. Morse, ed. also provides a comprehensive overview in Emerging Viruses, New York: Oxford University Press, 1993.
Y. Baskin discusses the role of human activities in creating new disease vectors in the tropics ("The Work Of Nature," Discover Vol. 16, No. 8, Aug 1995).
The Rainforest Action Network (RAN 1994) estimates the death rate from malaria among the Yanomani in Brazil and Venezuela at 20%.
Martin and Lefebvre raise the concern that global climate change will impact the distribution of malaria in "Malaria and climate: sensitivity of malaria potential transmission to climate," Ambio Vol. 24 No. 4, June 1995, while Binder et al. estimates malaria pediatric fatalities in Sub-Saharan Africa in "Emerging infectious diseases: public health issue for the 21st century," Science Vol. 284, No. 5418 (1311-1313) 21-May-1999.
According to Binder et al., infectious disease is the leading cause of death worldwide and the third leading cause of death in the United States ("Emerging infectious diseases: public health issue for the 21st century," Science Vol. 284, No. 5418 (1311-1313) 21-May-1999).
The U.S. Centers for Disease Control (CDC) reported to a congressional committee in 1997 that 10% of people who died before the age of 50 in 1994 did so suddenly and mysteriously possibly from some unidentified infection. In addition, the CDC noted that the U.S. spent only $42 million annually on infectious disease surveillance.
In the World Population Profile: 1998 (U.S. Government Printing Office, Washington, DC, 1999), the U.S. Bureau of the Census revealed the sobering impact of AIDS in the developing world.
E. Hooper (The River, Boston: Little, Brown and Company 1999) provides an excellent overview of the theories on origin of AIDS. He discusses the merits each of these in the course of describing the OVP/AIDS hypothesis he has come to adopt. This hypothesis says AIDS originated from the contimanation of a live polio vaccine with a simian immunodeficiency virus (SIV) during the mid to late-1950s. The other leading hypothesis, that of a "natural transfer" between SIV-infected chimpanzees and humans, is promoted in a widely read paper by F. Gao et al. ("Origin of HIV-1 in the Chimpanzee Pan troglodytes troglodytes," Nature, Vol. 397 (436-441), 1999).
At the 7th Conference on Retroviruses and Opportunistic Infections in San Francisco, B. Korber announced that the Los Alamos National Laboratory had traced the divergence of AIDS from SIV to around 1930 (Korber, B. et al., "HIV Databases and Analysis Projects at Los Alamos: An Overview," 1/30/00). The study assumed genetic changes in the virus occur at a constant rate. Should this dating prove correct it would undermine OPV/AIDS hypothesis supported by Hopper 1999.